Why did the NTPC boiler explode at Unchahar?

On 1 November 2017, the NTPC had its biggest accident ever, when a boiler furnace exploded at the Unchahar thermal power plant in Uttar Pradesh. The death toll has risen to 37, while more than 80 people have been seriously injured. Newsclick pieces together the sequence of events, from talking to various sources, which led to the explosion.